Neutral pH

What is a neutral pH?

An aqueous solution is neutral when the concentration of hydrogen ions in solution is equal to the concentration of hydroxide ions in solution:

Neutral solution: [H⁺(aq)] = [OH^-(aq)]

The pH of a solution is a measure of the concentration of hydrogen ions in solution:

pH = -log₁₀[H⁺(aq)]

For example, an aqueous solution will be neutral if  [H⁺(aq)] = [OH^-(aq)] = 10^-3 mol L^-1

For this solution the pH will be: pH = -log₁₀[H⁺(aq)] = -log₁₀[10^-3] = 3

For example, an aqueous solution will be neutral if  [H⁺(aq)] = [OH^-(aq)] = 10^-6 mol L^-1

For this solution the pH will be: pH = -log₁₀[H⁺(aq)] = -log₁₀[10^-6] =6

"Neutral" is NOT a pH.

Why do so many (non- Chemists!) believe a solution is neutral if the pH=7 ?
An aqueous solution can have a pH of about 7 IF the temperature of the aqueous solution is about 25°C.

At 25°C, a neutral aqueous solution has pH≈7

IF the temperature is less than 25°C, the pH will be greater than 7

Temperature <25°C, a neutral aqueous solution has pH>7 

IF the temperature is higher than 25°C, the pH will be less than 7

Temperature >25°C, a neutral aqueous solution has pH<7

The pH of a neutral aqueous solution depends on the temperature of the solution!!!

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Suggested Study Questions:

1.  In an aqueous solution the concentration of hydrogen ions is the same as the concentration of hydroxide ions. Is this solution acidic, basic, or, neutral? 
2. In an aqueous solution, the concentration of hydrogen ions is 2.7 x 10^-6 mol L^-1 and the concentration of hydroxide ions is 2.7 x 10^-6 mol L^-1. Is this solution acidic, basic, or, neutral. Explain your answer.
3. The concentration of hydroxide ions in a neutral aqueous solution is 5.8 x 10^-6 mol L^-1. What is the concentration of hydrogen ions in this solution? 
4. The concentration of hydrogen ions in a neutral aqueous solution is  7.9 x 10^-7 mol L^-1 What is the concentration of hydroxide ions in this solution?
5. Calculate the pH of the solution in question 4.
6. Calculate the pH of the solution in question 3.
7. Calculate the pH o the solution in question 2.
8. Explain how you would calculate the pH of the solution in question 1, justifying any assumptions you need to make.
9. Critically evaluate this statement, "At pH 7 a solution is neutral".
10. Chris the Chemist tested the pH of the water in the local pool. Monday was a hot day, about 40°C, and the pH of water was 6.82. That night the temperature dropped dramatically and the next day was a chilly 10°C. When Chris tested the pH of the pool water it was found to be 7.59. Is the pool water acidic, basic, or neutral? Justify your answer.

What is the pH of a Strong Base After You Dilute It?

The problem with strong bases like sodium hydroxide and potassium hydroxide are that they absorb moisture from the air making it difficult to accurately weigh them. So we rely heavily on obtaining an accurately known concentration for an aqueous solution of a strong base (using titration techniques for example) and then diluting this stock solution to produce new, dilute solutions of known concentration and pH.
When you dilute an aqueous solution of a strong base :

• hydroxide ion concentration decreases (towards 10^-7 M)
• pOH increases (towards 7)
• hydrogen ion concentration increases (towards 10^-7 M)
• pH decreases (towards 7)

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